1. Field of the Invention
The present invention relates to an exhaust gas recirculation apparatus to allow part of exhaust gas discharged from an engine with a supercharger to an exhaust passage to flow as exhaust recirculation gas in an intake passage to return to the engine, and more particularly to a failure detection device configured to detect a failure of the exhaust recirculation apparatus.
2. Related Art
Conventionally, a technique of the above type is employed in a vehicle engine, for example. An exhaust gas recirculation (EGR) apparatus is arranged to introduce part of exhaust gas after combustion, which is discharged from a combustion chamber of an engine to an exhaust passage, into an intake passage as EGR gas through an EGR passage so that the exhaust gas is mixed with intake air flowing in the intake passage and returns to the combustion chamber. The EGR gas flowing in the EGR passage is regulated by an EGR valve provided in the EGR passage. This EGR can reduce mainly nitrogen oxide (NOx) in the exhaust gas and improve fuel consumption during a partial load operation of the engine.
Exhaust gas from the engine contains no oxygen or is in an oxygen lean state. Thus, when part of the exhaust gas is mixed with the intake air by EGR, the oxygen concentration of the intake air decreases. In a combustion chamber, therefore, fuel burns in a low oxygen concentration. Thus, a peak temperature during combustion decreases, thereby suppressing the occurrence of NOx. In a gasoline engine, even when the content of oxygen in intake air is not increased by EGR and a throttle valve is closed to some degree, it is possible to reduce pumping loss of the engine.
Herein, recently, it is conceivable to perform EGR in the entire operating region of the engine in order to further improve fuel consumption. Realization of high EGR rates is thus demanded. To realize the high EGR rates, it is necessary for conventional arts to increase the internal diameter of an EGR passage or increase the opening area of a flow passage provided by a valve element and a valve seat of an EGR valve.
Meanwhile, in case a failure occurs in an EGR apparatus, it may interfere with appropriate EGR control, leading to the occurrence of knocking in an engine or the deterioration in exhaust emission of the engine. Therefore, there is conventionally proposed a failure detection device configured to diagnose the presence/absence of a failure of the EGR apparatus, inform a driver of such a fact in case a failure is detected, and store the fact in a storage device.
JP-A-2011-252399 discloses a technique of the above type for failure detection. In this technique, an intake pressure sensor actually measures intake pressure in an intake passage when an EGR valve is controlled to open and close and also an expected intake pressure to be realized by the control to open and close the EGR valve is estimated. By comparison between an actual measured value and an estimated value of the intake pressure, it is determined whether or not a failure of the EGR apparatus is present. If the failure is determined to be present, the type of that failure is determined. Herein, if the actual measured value of the intake pressure is different from the estimated value, it means that the intake pressure did not vary even when the EGR valve was controlled to open and close. It is thus possible to determine that the EGR valve is failed.
Furthermore, it is well known to adopt an EGR apparatus of an engine with a supercharger. JP-A-2012-007547 describes an EGR apparatus adopted in an engine with a supercharger. This engine is provided with a supercharger consisting of a turbine placed in an exhaust passage and a compressor placed in an intake passage and driven by the turbine. In this EGR apparatus, an inlet of an EGR passage is connected to the exhaust passage downstream of the turbine and an outlet of the EGR passage is connected to the intake passage upstream of the compressor, whereby constituting a low pressure loop EGR apparatus.